I've had the same thing happen to me before too with the stock sms service on the phone. A lot of near misses. Upgraded to Handcent app and haven't had a problem since
I'm almost positive it is the latest update that caused it for me. I just updated my phone last week to the latest update (and therefore giving up my free 4g hotspot haha), and it has sent texts to the wrong person 3 times. Two of the times it happened back to back. I also checked the recipient before sending the second one, and it showed the correct recipient, but then proceeded to send it to the wrong person after I hit 'send'. Basically, being careful and checking the recipient won't even solve the problem... I really hope the Gingerbread update will fix this issue.